I've received several PM's regarding integration of aftermarket equipment with the original equipment. I hope this will help answer some questions.
I would like to start out by saying that I'm not a prefessional installer. I've been a car audio enthusiast since the late 80's, I've put in many systems and I'm fairly well versed on theory. If any pros read my posts and see any errors, please let me know. I don't want to put out any misinformation.
In car audio, radios normally put out 3 types of signal: speaker-level, unbalanced line level, and balanced line level.
Most radios have a small built-in amplifier which you hook up to the speakers via the color-coded speaker wires. This is speaker-level
output. If you wanted to hook it up to an external amplifier, and the radio had no RCA outputs, you would need one of two things...an amplifier which takes speaker-level input, or a line level converter to change the signal to line-level.
Unbalanced line level
is the RCA-style "preamp" signal which is typically used in car audio. It consists of a center conductor which carries the signal, and an outer shield which is usually a ground. The voltage of the signal is typically around 4 volts.
is the type of system used in the Element. It consists of a positive wire, which carries a signal, and a negative, which carries the same signal 180° out of phase. The advantage of this system is that any noise picked up by the lines is cancelled out by the two phases. The two signals are combined and are about 8 volts.
When we try to mix aftermarket equipment with OEM equipment, we are mixing 4-volt signals with 8-volt signals. When we hook up an aftermarket headunit to the factory amp, we are sending a 4-volt signal to an amp that wants an 8-volt signal. Anybody who has replaced the radio like this probably noticed that you have to turn the volume up to be able to hear it, and even then it sounds kinda mushy with no real punch. The only way to remedy this is to install a balanced line driver before the amp, but it would be better to just replace the factory amp, altogether. There is no real problem with doing it this way and it has been done many times...it's just not perfect.
Hooking up an aftermarket amp to the factory radio is where it gets tricky. Since the factory HU puts out 8 volts, some amps can't handle the input. A few amps are designed to take a balanced input, but you will have to do your homework to make sure. One option is to use a signal processor such as JL Audio's CleanSweep to tune the signal. Another option is to use a balun (pronounced 'baloon') to convert the signal. After doing some research, I found the PAC AOEM-HON20 which seems
to fit the bill. http://www.pac-audio.com/productDetails.aspx?ProductId=561&CategoryID=28
Another area that people seem to get confused is with the term "power antenna". They think that since the antenna doesn't move up and down, that it's not powered. This is incorrect. The EX, SC (and I believe
the LX as well) has an amplified antenna. If you don't hook up the power antenna lead on the wiring harness, you will get poor reception.
Well, sorry for the novel. If anybody wants to add anything or sees any errors, feel free to post them.